TUOLUMNE CITY, Calif. — A raging wildfire in Yosemite National Park rained ash on the reservoir that is the chief source of San Francisco’s famously pure drinking water, and utility officials Monday scrambled to send more water toward the metropolitan area before it becomes tainted.

Officials monitored the clarity of the Hetch Hetchy Reservoir and used a massive new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the city. The Hetch Hetchy supplies water to 2.6 million people in the San Francisco Bay area.

“We’re taking advantage that the water we’re receiving is still of good quality,” said Harlan Kelly Jr., general manager of the city’s Public Utilities Commission. “We’re bringing down as much water as possible and replenishing all of the local reservoirs.”

At the same time, utility officials gave assurances that they have a six-month supply of water in reservoirs near the Bay area.

So far the ash that has been raining onto the Hetch Hetchy from the biggest wildfire on record in California’s Sierra Nevada has not sunk as far as the intake valves, which are about halfway down the 300-foot O’Shaughnessy Dam. Utility officials said the ash is nontoxic but that the city will begin filtering water for customers if problems are detected. That could cost more.